Treatment of dementias of various types, such as but not limited to, Alzheimer's disease (AD), Parkinson's disease, Huntington's disease and other forms, continue to be unmet medical needs. Alzheimer's disease is the most common form of dementia, wherein loss of memory and other intellectual abilities are serious enough to interfere with daily living. Alzheimer's disease is an age-related neurodegenerative disorder characterized by progressive loss of memory accompanied with cholinergic neurodegeneration (Kar, S.; Quirion, R. Amyloid β peptides and central cholinergic neurons: functional interrelationship and relevance to Alzheimer's disease pathology. Prog. Brain Res. 2004, 145 (Acetylcholine in the Cerebral Cortex), 261-274.). This disease accounts for over 50% of all progressive cognitive impairment in elderly patients. The prevalence increases with age. Alzheimer's disease is classified by its severity as mild, moderate and severe. The pathological hallmarks of AD include neuronal dysfunction/death, accumulation of senile plaques extracellularly and neurofibrillary tangles (NFTs) intraneuronally. Several hypotheses have been put forth to explain the pathophysiology of this disease, including aberrant β-amyloid (Aβ) metabolism, hyperphosphorylation of cytoskeletal proteins, genetic predisposition such as mutations in genes coding for presenilin-1 and -2 (PS-1 and PS-2) and amyloid precursor protein (APP), apolipoprotein E genotype, oxidative stress, excitotoxicity, inflammation and abnormal cell cycle re-entry. However to date, none of these hypotheses is sufficient to explain the diversity of biochemical and pathological abnormalities in AD.
Two pathological hallmarks of AD are generally recognized: senile plaques composed of β-amyloid peptide 1-42 (Aβ1-42) and neurofibrillary tangles (NFTs) formed by abnormal polymerization of microtubule-associated protein tau (Walsh, D. M.; Selkoe, D. J. Deciphering the molecular basis of memory failure in Alzheimer's disease. Neuron 2004, 44(1), 181-193.). While the precise cause underlying AD-related memory loss and cognitive changes remains to be fully elucidated, there is evidence indicating that pathological assemblies of Aβ1-42 cause diverse forms of AD and that tau plays a role including in mechanisms leading to Aβ1-42-induced neurodegeneration. More recent evidence from studies using transgenic animals suggests that tau pathology exacerbates neurodegenerative and cognitive processes in the presence of Aβ1-42 (Oddo, S.; Caccamo, A.; et al. Temporal Profile of Amyloid-β (Aβ) Oligomerization in an in Vivo Model of Alzheimer Disease: a link between Aβ and tau pathology. J. Biol. Chem. 2006, 281(3), 1599-1604.). In addition to Aβ and tau, dysregulation of calcium homeostasis also plays an integral role in the pathophysiology of AD (Green, K. N.; LaFerla, F. M. Linking calcium to Aβ and Alzheimer's disease. Neuron 2008, 59(2), 190-194.). It is becoming evident that dysregulation of mitochondrial function and resultant altered cellular homeostasis increasingly contributes to the pathology of neurodegenerative diseases such as AD (Moreira, P. I.; Santos, M. S.; et al. Is mitochondrial impairment a common link between Alzheimer's disease and diabetes? A matter under discussion. Trends Alzheimer's Dis. Res. 2006, 259-279. Beal, M. F. Mitochondria and neurodegeneration. Novartis Found. Symp. 2007, 287 (Mitochondrial Biology), 183-196. Reddy, P. H.; Beal, M. F. Amyloid beta, mitochondrial dysfunction and synaptic damage: implications for cognitive decline in aging and Alzheimer's disease. Trends Mol. Med. 2008, 14(2), 45-53.).
Mitochondria play major roles in bioenergetics and cell death/survival signaling of the mammalian cell as they are ‘gatekeepers of life and death’. Mitochondrial dysfunction contributes to the pathogenesis of various neurodegenerative diseases with pathophysiological consequences at multiple levels including at the level of calcium-driven excitotoxicity. One of the primary mitochondrial mechanisms is the mitochondrial permeability transition pores (MPTP) that represent a multiprotein complex derived from components of inner and outer mitochondrial membrane. The pores regulate transport of ions and peptides in and out of mitochondria, and their regulation is associated with mechanisms for maintaining cellular calcium homeostasis. A deficit in mitochondria is the earliest feature of neurodegenerative diseases. One general characteristic of aging and neurodegeneration is an increase in the number of neuronal cells undergoing signs of apoptotic degeneration. A key role for this apoptotic process is attributable to the mitochondrial permeability transition pore, which provides transport in and out of mitochondria for both calcium ions and compounds with low molecular weight. It has been proposed that MPTP is a multiprotein complex with the outer membrane fragment including porin (a voltage-dependent ion channel), anti-apoptotic proteins of the Bcl-2 family, and the peripheral benzodiazepine receptor. The inner fragment of MPTP contains an adenine nucleotide translocator and cyclophilin, which may interact with proapoptotic proteins of the Bax family Inhibition of mitochondrial calcium uptake and/or blocking of MPTP may protect cells against the development of apoptosis in the presence of pathological factors such as excitotoxins and anti-oxidants. Indirect modulation of MPTP via kinase pathways is also known wherein glycogen synthase kinase-3β (GSK3β) mediates convergence of protection signaling to inhibit the mitochondrial MPTP (Juhaszova, M.; Zorov, D. B.; et al. Glycogen synthase kinase-3β mediates convergence of protection signaling to inhibit the mitochondrial permeability transition pore. J. Clin. Invest. 2004, 113(11), 1535-1549. Juhaszova, M.; Wang, S.; et al. The identity and regulation of the mitochondrial permeability transition pore: where the known meets the unknown. Ann. N.Y. Acad. Sci. 2008, 1123 (Control and Regulation of Transport Phenomena in the Cardiac System), 197-212.) and mitochondrial localization during apoptosis (Linseman, D. A.; Butts, B. D.; et al. Glycogen synthase kinase-3β phosphorylates Bax and promotes its mitochondrial localization during neuronal apoptosis. J. Neurosci. 2004, 24(44), 9993-10002.). Furthermore, calcium-dependent activation of MPTP in brain mitochondria enhances with age and may play an important role in age related neurodegenerative disorders.
Neuroprotective effects of agents have been linked to various cellular processes including inhibition of mitochondrial MPTPs. For example, the neuroprotective effects of 4-azasteroids parallel the inhibition of the mitochondrial transition pore (Soskic, V.; Klemm, M.; et al. A connection between the mitochondrial permeability transition pore, autophagy, and cerebral amyloidogenesis. J. Proteome Res. 2008, 7(6): 2262-2269.). In vivo administration of MPTP inhibitor, 1-(3-chlorophenyl)-3-phenyl-pyrrole-2,5-dione to a mouse model of multiple sclerosis significantly prevented the development of the disease (Pelicci, P., Giorgio, M.; et al. MPTP inhibitors for blockade of degenerative tissue damages. WO 2008067863A2). Compounds such as dimebolin (latrepirdine, 2,3,4,5-tetrahydro-2,8-dimethyl-5-[2-(6-methyl-3-pyridinyl)ethyl]-1H-pyrido[4,3-b]indole) have been shown to improve neuronal function and a role for improved neuronal outgrowth and mitochondrial function has been suggested. Dimebolin has been shown to inhibit neuronal death in models of AD and Huntington's disease, another neurodegenerative disease (Lermontova, N. N.; Lukoyanov, N. V.; et al. Dimebon improves learning in animals with experimental Alzheimer's disease. Bull. Exp. Biol. Med. 2000, 129(6), 544-546. Bachurin, S.; Bukatina, E.; et al. Antihistamine agent dimebon as a novel neuroprotector and a cognition enhancer. Ann. N.Y. Acad. Sci. 2001, 939 (Neuroprotective Agents), 425-435.). More recently, dimebolin has been shown to possess a clinically beneficial effect in cognition in patients with AD (Burns, A.; Jacoby, R. Dimebon in Alzheimer's disease: old drug for new indication. Lancet 2008, 372(9634), 179-80. Doody, R. S.; Gavrilova, S. I.; et al. Effect of dimebon on cognition, activities of daily living, behaviour, and global function in patients with mild-to-moderate Alzheimer's disease: a randomised, double-blind, placebo-controlled study. Lancet 2008, 372(9634), 207-215.). Patients with mild-to-moderate Alzheimer's disease administered with 20 mg three times a day (60 mg/day) showed significant improvement in the clinical course of disease, as reflected in improvement over baseline for ADAS-Cog (Alzheimer's disease assessment scale—cognitive subscale). In particular, dimebolin-treated patients demonstrated a significant improvement over placebo in cognition, global function, activities of daily living and behavior. A six-month open-label extension trial of dimebolin produced results similar to those in the preceding 12-month clinical trial (Cummings, J.; Doody, R.; Gavrilova, S.; Sano, M.; Aisen, P.; Seely, L.; Hung, D. 18-month data from an open-label extension of a one-year controlled trial of dimebon in patients with mild-to-moderate Alzheimer's disease. Presented at the International Conference on Alzheimer's Disease (ICAD), Chicago, Ill., USA, July 2008; paper P4-334.). Patients with mild-to-moderate AD who had earlier received the drug for 12 months had preservation of function close to their starting baseline on key symptoms of AD. Patients originally on placebo who received dimebolin in the extension study showed stabilization across all key measures.
Dimebolin has been approved in Russia as a non-selective antihistamine. The drug was sold for many years before selective anti-histaminergic agents were developed. Although dimebolin was initially thought to exert its cognitive enhancing effects through inhibition of butyryl-cholinesterase, acetyl cholinesterase, NMDA receptor or L-type calcium channels (Bachurin, S.; Bukatina, E.; et al. Antihistamine agent dimebon as a novel neuroprotector and a cognition enhancer. Ann. N.Y. Acad. Sci. 2001, 939 (Neuroprotective Agents), 425-435. Lermontova, N. N.; Redkozubov, A. E.; et al. Dimebon and tacrine inhibit neurotoxic action of beta-amyloid in culture and block L-type Ca(2+) channels. Bull. Exp. Biol. Med. 2001, 132(5), 1079-83. Grigor'ev, V. V.; Dranyi, O. A.; et al. Comparative Study of Action Mechanisms of Dimebon and Memantine on AMPA- and NMDA-Subtypes Glutamate Receptors in Rat Cerebral Neurons. Bull. Exp. Biol. Med. 2003, 136(5): 474-477.), its interactions at these targets are weak. More recent data suggest that dimebolin may exert its effects at the level of mitochondria, and that these activities could enhance neuronal function (Hung, D. Dimebon: A phase 3 investigational agent for Alzheimer's disease with a novel mitochondrial mechanism of action. Presented at the International Conference on Alzheimer's Disease, Chicago, Ill., USA, July 2008; paper S4-04-05.). Hung and coworkers (Hung, D. Dimebon: A phase 3 investigational agent for Alzheimer's disease with a novel mitochondrial mechanism of action. Presented at the International Conference on Alzheimer's Disease, Chicago, Ill., USA, July 2008; paper S4-04-05.) reported that dimebolin can protect cells from excitotoxic damage and improve neurite outgrowth in neuroblastoma cell lines and primary neurons. From an adverse effect standpoint, in recently reported clinical studies of dimebolin, the most frequent adverse event was dry mouth, which is consistent with the antihistaminic effects of dimebolin (Doody, R. S.; Gavrilova, S. I.; et al. Effect of dimebon on cognition, activities of daily living, behaviour, and global function in patients with mild-to-moderate Alzheimer's disease: a randomised, double-blind, placebo-controlled study. Lancet 2008, 372(9634), 207-215.). There is a need in the art to identify and provide novel agents for treating or preventing conditions associated with neurodegenerative disorders such as AD, lacking histaminergic (H1) interactions.
As noted earlier, given the likely multiple etiologies of neurodegenerative diseases such as AD, multiple avenues are being pursued as symptomatic approaches or as disease modifying approaches to alter the underlying pathology of the disease (Scatena, R.; Martorana, G. E.; et al. An update on pharmacological approaches to neurodegenerative diseases. Expert Opin. Invest. Drugs 2007, 16(1), 59-72.). In particular, the reported benefit of dimebolin in double-blind, placebo-controlled study of patients with mild-to-moderate AD across many cognitive and clinical measures demonstrates the potential of such compounds to prevent or treat a variety of neurodegenerative diseases where an underlying pathology involves deficits in cognitive function. In addition to the need for improved receptor selectivity profile (as for example vs. H1 receptors), one of the current limitations with dimebolin is the dosing regimen necessitating three times per day (t.i.d.) administration in humans. As neuroprotective approaches exemplified by dimebolin continue to be validated as a viable clinical approach, there is a need in the art to identify and provide novel compounds for treating or preventing cognitive deficits associated with AD and other neurodegenerative and neuropsychiatric diseases.